It is often convenient for construction and repair service work that materials, equipment and personnel be raised at a work site to elevations in the range of 30 to 40 feet by use of a portable lift that can be wheeled when lowered through doorways, and can be easily transported. For stability when elevated, it is essential that such lifts have outriggers, and it is preferred that the outriggers be easy to move between active and stowed positions. It is also preferred that the outriggers have an arrangement permitting the work platform of the lift to be positioned close to walls and corners. Lifts of this general type are disclosed in U.S. Pat. Nos. 4,015,686 and 4,458,785, for example.
Commonly, such lifts have a mast extension system operated by extension of a hydraulic cylinder which is supplied with pressurized fluid by a pump powered by an electric motor, and having its suction connected to a reservoir tank. The pump, motor, and tank are mounted on the frame of the lift, and suitable controls are provided so that the work platform can be raised and lowered by a person occupying the work platform or at ground level.
However, a need exists for a safety system which prevents raising the work platform unless the outriggers are fully extended and properly positioned in a load bearing condition.